CHAPTER Surface Modification and Surface...

CHAPTER -5

Surface Modification and Surface Melting

5.1 Introduetiori

Engineering the surfaces of components used in automotive and aerospace

engineering to improve their lives and perfoi-mances is the active area of research

Suitable Ther~nali Mechan~ca1,"Thenno chemical surface engineering treatineilts will

produce extensive rea~~angement of atoms in metals and alloys and corresponding

~narked variations 111 Physical, Chemical and Mechanical propelt~es. .Among the more

linportant of these treatments are heat treatment processes such as hardening by

Quenching, Illduclion hardening and Case Carburizing which rely on phase

tlnnsformations to producc dcslrcd changes in mechanical piopel-tles. Other processes

\vhe~-e phase transformations occur are cast~ng, welding and machining etc.

In industry, different heat trzatinent methods are used for production of reilulred

surface layer properties. A classification of the surface liardenlng methods based on three

main mechanisms viz., mechanical, thermal and thei-ino chenlical alterations of surface

layers is given in figure 5.1. It is clear from the classification that surface layer hardening

by lnachining remains an unestablished method of obtaining the required mechanical

propelties on the component surfaces (Brinksmeier et al.. 1997). With the exceptioil of

some investigations into surface layer hardening by friction hardening, a gap exists in the

development and use of surface layer hardening by machining.

Based on the chip removal and chip formation mechanisms, it is understood that a

substantial pait of cutting energy in i~lachining is transformed into thermal energy. The

dependence of theimal loading of the workpiece due to the development of heatflux at the

surface layer on the machining conditions and type of machining process was first

described by Snoeys and Maris (1978). The process parameters and the thermo-physical

properties of the work and tool inaterials principally influence the effective work surface

temperature. if the material in f h i s~lsface layer is heated above the chasacreristics

temperature (720°C - 910°C) ciuring lnachiliing operation, diffusion and phase

transformation take place. Thus. most of the research findings show that the mechanical

and metallurgical characteristics of machi~ied surfaces can be contl-olled by controlling

tile effective \vol.k si~rface temperatut.o.

Established methods

studied methods

O unestablished methods

Figure 5.1 Surface strengthening processes,

With this in mind. in this WOI-li a study has been made in using the the]-ma1 energy

generated in grinding for obtaining required hardness ievei at the surface layer without

having adverse effect on the quality of the job. In this study. the surface metallurgy o f the

ground hardened con~ponents. turn hardened and conventionally surface hardened

components have also been analyzed and compared to get a better understanding on this

phenomenon.

5.2 Sul-face hardening processes

4 thorougl~ knoti8iedge of'\ nrious su~.i'aci. hardriling PI-uccsscs hslps the engineers

and researchers in deciding the type o f process to be employeti to obtain requirctl surface

characteristics for the efficient and smooth p e r h n n a ~ ~ c e o f the engineering con1poilents

in their allotted functions. Surface hardening can basically be achieved by two ciifiirent

methods. The ail11 of both the methods is same. The first method is thel~nochemical

surface hardening, in which the surface composition o f the steel changes by ditiusion o f

carbon and or nitrogen 01- sometimes by other elements. Carburizing. Cyaniding,

Carbonitriding. Plasmanitriding. Boronizing. Chromizing and Toyota diffusion process

are some o f them. The Second method is Theinla1 impact surface hardening which

involves phase transfo~mation by rapid heating and cooling of the outer surface (Flame

hardening, Induction hardening, Electron Bean1 hardening, Laser surface hardening and

Salt bath hardening). In Majority of the industries, the gas carburising and induction

hardening processes are very common (Rajan et al., 1991). Hence, the objectives and

principles of the Inductioa hardening and gas carburising processes are discussed below:

5.2.1 Inductiorl hardening

Generally this process is used to surface harden crank shaft, call1 shall, gears,

crank pins and axles. In this process heating o f the components is achieved b y

electromagnetic induction. A conductor (coil) c a l ~ i e s an altelnating current of high

frequency which is then induced in the enclosed steel part within the magnetic field o f the

coil. As a result, induction heating taltes place. The heat so generated affects only the

outer surface of the steel components due to skin effect. T h e component is heated usually

for a few seconds only. immediately the surfhce i.s quenched b!. thejet of cold water. Due

to cluenching a martensitic strucl~ire is Ihl-med \ihich makes tile outer surfhce hard and

\\,ear resistance. The hardening temperature ranges from 760 to 930. OC. Accorciing to

the Carbon content and alloy addition. the temperature is fixed (Palaniradja et al.. 2005).

5.2.2 Gas carburising

Carburising is the niost widely used process for surface hardening of steels. In this

process, carbon is diffused into steel by heating above the transformation temperature and

holding the steel in contact with a carbonaceous niaterial. wliich may be a solid medium.

liquid. or a gas. Under such condition, carbon is absorbed in solid solution in austenite.

As the solubility of carbon is more in the austenitic state than i l l fen-itic state. fully

austenitic steel is essential for carburising. Carburising can be divided into Pack

carburising, Liquid Carburising and Gas Carburising. Among them Gas Carburising is

the nlost widely used industrial heat treatment process.

Gas carburizing is a process in which the surface of the conlponents is saturated

with carbon in a gaseous atmosphere containing carbon. To accomplish this. first the

components are heated in a gas tight funlace in a neutral atmosphere to a predetermined

temperature in the range of 870 O C to 940 "C. Then the furnace is flooded with a suitable

gas such as Propane, Butane, and Kerosene etc. Finally the conlponents are held at this

temperature to allow diffusion of carbon into the case. After the carburizing treatment is

over the components are quenched to get the required hardness, wear resistance and

fatigue resistance on the surface, supported by a tougher core. A striking feature of gas

carburizing process is that in this process the original toughness and the ductility

remains unaffected even after the heat treatnlent (Palaniradja et al., 2005).

5.3 Grinding and heat-treating

Many research findings indicate that there is definite evidence that during

grinding the workmaterial is subjected to conditions akin to the heat treatment process. It

is because of the fact that in grinding the surface temperature of the work lnaterial rises in

the range of 1000°C and 1700 "C that too with shai-p heat gradient. This can have

considerable effect on the surface integrity of the workpiece. As the wheel abrasivss are

insulators. most of the heat generated during gsinding goes into the work piece. M%en

fine grinding with A120;. without a coolant. about 80% of the total heat energy ends up in

the workpiece (Sato, 1961; Malkin. 1978). This is because of the lllaterinl removal

mechanism involved in gl-inding. The ploughing aclion before the separation of chip

causes higher late of strain due to deformation and which in turn colltributes to the

generation of heat that too at a distance ecjual to the depth of cut from the s1;in of he

\iorkpiece.

111 grinding, virtually all mechanical energy is converted into the thermal tnergy.

heat. The heat conducted into the worl

5.4.1 Experimentation in Gas carburising and Induction hardening

To compare the surface metallurgy of the conventionally surface ha-dened

matesial with the machine-hardened material. trials are cai-ried out in Gas Carburising and

Induction hardening furnaces. The conditions undei-m hich tlie trials have been ca~r ied out

and the results obtained fi-om the above-ment~oned trials are given in Tables 5. I . 5.2 and

Table 5.1 Details of Gas carburising --

I Material Used . AISI 33 10 - Lou Carbon atcel Diameter : 1 jmm ; Length : 200 inln I Fuinace Details :

I ! Methanol - Acetone Unithe~m Gas Carburising Furnace of 3 /z in depth ~ Electi-ical rating : I30 K W

I Temperature : 870°C to 940 "C

Operating conditions: I Holding time - 200 minutes (maximum) Quenching time - 90 ininutes (maximum) Carbon potential - I 1 1 OmV.

Table 5.2 Details of Induction hardening

Material Used : AISI 4140 - hledium Carbon steel

Diameter : 25mm ; Heating length : 1501nin I

Furnace Details :

440V, 31nm coupling distance Inducto Heat induction harclening device

Frequency : 1000 to 10.000 cycles per second

Temperature : 750 to 800°C. I

Operating conditions: i Power Potential - 5.5 kw/inch2 Scan speed - 1.72 mlminutes I

Quench Flow rate - 15 Litei-slminutes I

Table 5.3 Micro hardncss \values of sul-face hardened materials

, Depth beneath the surface 1 Average Value of Micro ha1-dness in VHN ! in mm

/ Gas Carburized (33 10) Induction hardened (4140)

*Even though. a total case depth of upto 4 mm is possible in the con\entionaI

surface hardening processes, the measurement of microhardness is restricted to 1.92mm

only. Since higher case depth is not possible with inachine hardening.

5.4.2 Experimentation in Rough turning (turn hardening)

Tu~ning is a process that removes excess material by a wedge -shaped tool to

generate specified geometry and surfaces. Turning process itself is uniquely characterized

by high stresses, high strain rate, high temperature, and generally short interaction time

with the workpiece malerial encountered during chip formation. Thus, tunling process

alwaj~s results in some metallurgical changes at the workpiece surface, such as

niicrostructural alteration, microhardness changes. and residual stresses. These changes,

which are coming under surface integrity classification, are impol-tant, especially in

finishing processes: for they affect the performance of the workpiece in its allotted

function. An investigation has been conducted to identify the potential of applying heat

energy generated in turning to modify the surface layer of steel parts. The conditions

undemlhich the tu1-n hardening experimentation are carried out given in the Table 5.4 and

5.5. The results are reported in the Table 5.6.

Table 5.4 Details about materials subjected for rough ti lrnit~g test

Table 5.5 Lathe Machine details and the operating parameters

S.So

I 1

7

1 i .I L

Machine : HMT Lathc Model : NH 26 Cutting tool signature : 0.7mm radius, 5" Clearance angle

and -30" Rake angle Cutting conditions : Depth of cut 1000,1200 and 1400pm Cutting speed : 3-5m:sec Feed rate : 0.15 -0.25inni/rel~. Length of cut per pass : 90- 100mm

Material

Medium carbon st~uctural steel Diameter

=3 51111n Medium carbon spring steel I

High carbon liigb specd ~ A I S I T ~ 1 O R O

I

I 5On1m steel I

Table 5.6 Micro liard~less values of the turn-hardened materials

5.4.3 Experimentation in Rough grinding (Grind hardening)

Rough grinding is carried out as per tile conditions given. under Table 5.7 and 5.8

and the results are tabulated in the Table 5.9.

Table 5.9 Micro hardness values of grind hardened materials Depth beneath the I .4\erage Value of Mlci-o haidness of ground specimen for various surface in mill depth of cut , ~n VHN

AISI 4140 I .&IS1 9255 AISI T4 I

1000 1200 1400 1 1000 1200 1400 1000 1200 1400

0 48 1

Table 5.7 Details about materials subjected for rough grinding test

S.No

I

Type of wheel used

Size

Mediuln carbon spring 2 ~ steel

Material 9'0 of

P.120: Diameter

I =35mm

AlSl 9255 0.55 X1203 and Lengtli =

Medium carbon structural

-...- steel

, High carbon high speed 1 7

I steel .41SI T4 150mrn

Table 5.8 Grinding conditions (Rough grinding) 1 Machine: CGU 500 Vlodel His11 Precision plunge type c);lindrical I

grinding machine. Grindii~g wheel speciiicarioli :WPsel diaineter -35Omni and \vidth- 60mni ~ Cutting conditions : Depth of' cut 1000.1200 and 1400pm Cutting speed of the wheel : 251n!sec

1 Cutting speed of the work piece : 1 misec L Number of passes : Ranges from 5 - 10

' Designation Carbon

AISI 4110 0.40 _-

+ In tul-n hardening and grind l~nsilening tile 1~1ic1.o liarclness \,ariation is upto a niasi~iiun~ of2111111 beneath the surface and 11ence i t is measul.ed upto 1.921iirn. Table 5. 10

s h o ~ s the maxin~uiil and minimu~n hardness obtained in the different surface

modification processes for different AISI steel materials subjected for experimentation

Table 5.10 Maximum and Mininlum micro hardness obtained in the different surface modification processes

1 3 Induction ~ 630 1 210 hardened I 1 1.4mm I

/ Nameof the 1 Maximum S.NO. i material micro hardness , inVHN

1 AIS14140 236

Minimum micro hardness in \'HN

210

i

.At n depth of 1

Remarks

At a depth of 1400 pin T u ~ n hardened

AISI 4140 I Grind hardened

3 15

AISI4140 I

1 AISI 9255 1 241 1 I 215 Tum hardened I I

cut 1200 p1n 1 At a depth of cut 1400 pm At a depth of cut 1200 p 1

210 At a depth of

, ' Case depth AISI 3310

54 1 220 1 maximum Gas Carburised 1 5611-1111

I

215

230

230

5

'

5.5 Surface modification -A comprehensive study

Long back, grinding was named as an unintentional heat treatment method.

AISI 9255 365

Grind hardened 1 AISI T4 305

Turn hardened

Recently, the idea was taken up and fundamental investigarions were call-ied out to

AISI T4 Grind hardened

develop suitable strategies of process control for well-aimed modifications of surface

3 86

layer (Komandurai, 1993)

In finishing operatio11 of engineering components, grinding process tinds an

important place because they allow the machining of both hard and brittle materials to

high dimensional and geo~netric accuracies with a good surface finish (Hahn: 1962).

In general. engineering colnponents have to be subjected to a hardening heat

treatment process at the surface layer to ha\,e both fatigue strength and wear resistance

properties. Highly loaded components subjected to relative inotions are often surface

hardened using a variety of processes based on thel-ma1 01- the]-mo-chemical impact. e.g.,

Indllction or Case hardening. Such heat treatment processes. which are restricted to the

surface layer. arc connected with some typical ad\,antagcs compared to ti111 hardening

lieat treatments. In industry, dif'ferent heat treatlnent methods arc used for the production

of required surface layer properties. The problem is that these processes cannot simply

be integrated into the production line causing economical disad\.antages.

Further, surface hardening is a critical process due to its effects on production cost

and part quality. It also has greater influence on energy efficiency and inipact on the

environment that receive fast growing attention. Depending on part dimension,

geometry, and batch size, the cost of surface hardening varies and it can have

considerable effect in its applications. Moreover. the cycle time and the associated down

time of surface heat treatment substantially lower productivity v/hich may rum out to be

the bottleneck to process flow optim.ization. The proposed new approach is to utilize

grinding process for surface hardening also while it is pertbrming its rllajor function as a

material removal process. The propoxci surface tnodification approach by grinding

provides possible benefits such as higher productivity and lo\ver production cost.

The superimposition of theirnal and mechanical load is able to cause alterations in

the surface layer like, tempered zones, white etching areas etc. These effects were

investigated and discussed by Field and Kahles (I97 1).

The process parameters and the thei-mo physical properties of the work and tool

materials principally influence the effective work surface temperature. If the material in

the surface layer is heated above the characteristics temperature (870°C - 9 10 "C) during

machining operation, diffusion and phase tiansfoimatio~l take place. So. most of the

research findings show that the surface integrity of machined surfaces can be controlled

by controlling the effective work surface temperature.

In any case, [lie generated lleat in grinding is considzrecl as a restricting Ikctor that

necessitates the use of coolants and the ssjection of moderate grinding conditions. The

abo\.e said limitation causes one to investigate how this process energy can be effectively

controlled and utilized to improve the surface integrity and also prevent thelmal dainages.

To obtain this. attempt is made with different steel materiais for different grinding

conditions. The experilnental obsz~liations are tabulated (Table 5.1 1 to 5.28) and the

corresponding characteristics curves are drawn (Figures j . l i to 5.30). The

~nicrostructures of \:arious surface ~nadified AlSI steel materials which are sul~jected to

grinding process are also reported (Figures 5.2 t05.10).

In this experiment, the forniation ot'a heat-trtateci zone (,HTZ) beneath the surface

is characterized by a significant hardness increase. This heat treated zone is compclscd of

t\vo different boundary layers - The surface upto a few micrometer thick ~vhite etclling

area with an extremely high hardness followed by a hardened st~ucture consisting of

martensite and carbides. The interesting observation or fact is that in tile experi~nents

conducted no time surface cracks have been developed on the grind hardened parts.

.Table 5.11 Details about materials subjected for grind hardening test

3

4

Medium c a ~ b o n st~uctulal steel

Med~uin carbon s t ~ u c r u ~ a l steel

1 5 Med~urn carbon spring steel AISI 9255 0 55 and 1 Lengrh = 15 Omni

8 High carbon tool steel AISI 1095 0 90 A1203 1

AlSI 1040

1 High carbon non shnnklng

die steel

0 35 A120:, I

AISI D2

Diameter ~ =35mm 1

AiSl 4140 , 40 I

1 70 A1203

A120;

1

A1203

Table 5.12 Hardness a t various depths for different depth of cut for AISI 3310

beneath the Total depth of cut with varying number of passes, in s.No. surface in mm 1 I 'HN H 1400

I

Table 5.13 Hardness a t various depths for different depth of cut for AlSI H21

Depth I

Rlicro hardness values for different

1 1 Depth I Micro hardness values for different 1 1 beneath the Total depth of cut ~ v i t h varying number of passes, in

S.No. 1

Table 5.14 Hardness a t various depths for different depth of cut for AlSI 1040

8

Micro hardness values for different Total depth of cut with varying number of passes, in ~

S.Uo. surface in rnrn / V H N I I Dc / 100 600 SOU 1 1000 I 1200 / 1100 1

Table 5.15 Hardness a t various depths for different depth of cut for AISl 1095

Depth Micro hardness alues for different I beneath the Total depth of cut with varying number of passes, in

S.No. surface in mm 'VHN I ' Dc 400 600 800 1 1000 1200 1400

Table 5.16 Hardness a t ~ a r i o u s depths for different depth of cut for AISI 01

Table 5.17 Hardness a t various depths for different depth of cut for AISI D2

-

S.YO. I

I Micro hardness values for different 1

depth of cut with varying number of passes, in VHK

400-T00 800 1 1000 1200 1 1400

Depth beneath the

surface in rnrn DC

Micro hardness values for different Total depth of cut with varying number of passes, in

VHN I 3 4 T I

Table 5.18 Influence of number of passes on hardness and surface roughness for AISI 3310

Table 5.19 Influence of number of passes on hardness and surface roughness for AISI H21


S,No.

1

/ S.No. 1 Number of 1 Maximum / Ra ,pm ! Rt, pm I Rz, p111 1

Number of passes

5

Maximum hardness

3 14

Ra, pin ' Rt, 1-lm 0.21 2 . 2 0

Rz ,pi11

2.50


Table 5.22 Influence of number of passes on hardness and surface roughness for AISI 0 1

S.No.

/ S.No. 1 Number of I Il.laximum / Ra, IJm / Rt, pln 1 Rz, pm 1

Number of passes

Table 5.23 Influence of number of passes on hardness and surface roughness for AISI D2

I J 1 J lJV ( CI.AI 1 . 0 I L.00

2 7 3 17 / 0.19 2.30 2.52 , '

passes / hardness 1

1 S.No. 1 Number of / Maximum 1 Ra, pm Rf, lim : Rz, Pm 1

Nlaximurn hardness

4 5 6

I

Ra, pm

1 1 13 I5

1

1 2 3 4 5 6

Rt ,pm / Rz ,pm

5 270 I 0.18 / 1.89 / 2.62 1 I

3 54 367

passes -- 5 7 9 11 13 15

0.17 0. 16

hardness 34 1 3 74

-- - 2.80 _ 2.89 2.63

I

319 / 0.25 / 3.02 / 3 .11

2.24

7 0.19 2.22

396 0.17 2.68

- 2.57 ~ 2.32 2.84

434 , 0.15 1 3.01 2.24 477 0 . 1 7 2.48 , 2.74 424 0.22 3.05 3.30

Table 5.24 Influence of infeed rate on hardness a t various depths for different depth of cut AISI 4140

Table 5.25 Influence of infeed rate on hardness a t various depths for different depth of cut AISI 9255

I Depth beneath S.No. the surface in

Micro hardness a t different total depths of cut with varying number of passes, in VHN

I

I Micro hardness at different total depths of cut with I

Depth beneath varying number of passes, in VHN S.No. the surface in 1

I m m 2.3n1mimin

Table 5.26 Influence of infeed rate on hardness a t various depths for different depth of cut AISI T4

/ Micro hardness at different total depths of cut with Depth beneath 1 varying number of passes, in VHN the surface in

Table 5.27 Influence of infeed rate on hardness penetration depths for different AISI materials

Table 5.28 Influence of % of carbon on hardness for the AISI steel materials

I I I

S.No. 1

3 4

6

Material

AIS14140

I I 0.6 I 1.12

AISI 9255 ~ 1.25 1.12 2.3

7

9

Infeed in mmlmin 0.6 1.25

1.26

Hardness penetration depth, in mm 0.98 1.26

2.3

I 0.6

1.26

1.26

AISI T4 I 1.25 I 1.26 2 3 1.40

Figure 5.3 Microstructure of AISI H21

Figure 5.6 Microstructure of AlSI 9255

Figure 5.7 Microstructure of AISI T4

Figure 5.8 Microstructure of AISI 1095

Figure 5.10 Microstructure of AISI D2

Depth beneath the surface in rnm

Z 245 1 240 -

zxlnnn"1200 1 4 0 7

-5 235 - % 230 - a,

225 - k 220 - .c 2 215 - .Y 210 --- I I

I 0 0.5 1 1.5 2 2.5 Depth beneath the surface in mm

Depth beneath the surface in mm

Figure 5.11,5.12,5.13 Micro hardness at various depths for different total depth of cut of turn hardened materials




Figure 5.14,5.15,5.16 Micro hardness at various depths for different total depth of cut of grind hardened materials

800 -- > --- &Gas C a r a ~ r l s e d I

600 A - 2 - m -d-= > a-z-z Depth beneath the surface in mm

Figure 5.18 Micro hardness Vs depth beneath the surface for different total depth of cut (AISI 3310)


Figure 5.19 Micro hardness Vs depth beneath the surface for different total depth of cut (AISI H 21)



Depth beneath the surface in mrn





Figure 5.23 Micro hardness Vs depth beneath the surface for different total depth of cut (AISI D2)

' + AISI 3310 1

Number of Passes

Figure 5.24 Influence of number of passes on hardness

Total Depth of cut in mm

Figure 5.25 surface roughness values for different depth ground (AISI 1040)

0 0.5 1 1.5


Figure 5.26 Hardness at various depths for different total depth of cut for an infeed rate of 0.6 mmlmin (AISI 4140)


Figure 5.27 Hardness at various depths for different total depth of cut for an infeed rate of 1.25 mm/min (AISI 9255)


Figure 5.28 Hardness at various depths for different total depth of for an infeed rate of 2.3 mmlmin (AISI T4)

& 1.5 -1 E 1 C

e - 1 1 -- - - .- 2 1 'a El

-"AIS14 140 0 .e I u

+AISI 9255 0.5 -

L AISI T4

u Q)

E P, rn 0 -I - --- - --- - - Y) : 0 0.5 1 1.5 2 3 2.5

Feed rate in mm/minutes

Figure 5.29 Influence of federate on Hardness penetration depth for the materials AISI 4140,9255, T4

_I AlSl D2 1 AISI 9255 AlSl 1095

1 AISI 3310 AISI 4140

Carbon percentage

Figure 5.30 Influence of percentage of carbon on Hardness for the materials AISI 3310, H21,1040,4140,9255, T4,1095,01, and D2

5.6 Surface melting in grinding

Heat generated in the contact area between a wheel and work piece (grinding

zone) is the main cause for the deterioration in the ~netallurgical propel-ties of the work

piece, dimensional accuracy and wheel life ( Shaw, 1984a) In order to estimate and

predict these damages due to thermal effect, nolnlal and ab~loimal grinding conditions

have been investigated experi~nentally and reported hel-e.

One of the first attelnpts to calculate grinding temperatures \vas rt'portzii by

Out\vater and Shaw (1952). 111 this analysis it was assunled that all the grinding energy

uJent into chip formation.

4 somewhat different approach to the calculation of grinding tempcl-ati11.e was

taken by Sato (1961). The surface tenlperature was calculated using the concept of an

Instantaneous heat source.

Malkin (1984) combined the concept of both a local and average gi-inding

temperature in order to evaluate the surface temperature distribution in grinding. The

local grinding temperature refers to the temperature rise on the workpiece surface due to

the cutting action of an individual abrasive grain; there is also an additional temperature

rise due to the grinding action of all the other abrasive grains in the grinding zone. This is

called grinding zone temperature.

The grinding temperature is calculated as the sum of a local temperature in the

vicinity of an abrasive grain and a grinding zone temperature over the apparent area of

contact between the grinding wheel and the work piece (Shaw, 1990). The peak local

temperature at the cutting edge of an abrasive grain is found to be close to the melting

point of the work piece. It reveals that, work piece bum occurs at a critical grinding zone

temperature.

For grinding of various steels, burning has been found to occur when a critical

wear flat area is reached. The magnitude of which depends upon the operating conditions

and the particular steel being ground. Before continuing to grind, it is necessary to redress

the wheel. In this way work piece burn can detelmine the grinding wheel tool life (Tool

life refers the time period between successive dressings). Therefore, the grinding

conditions under which work piece bum will occur are of practical significance. In this

connection four AISI steel materials shown in table 5.29 are subjected for surface melting

study by grinding them under abusive grinding conditions (Total depth of cut - between

1200 microns and 1600 microns with number of passes 4 to 6).

Table 5.29 Details about materials subjected for Surface melting study

Under abusive grinding conditions, thermal damages occur. However, under

optimal conditions, process energy generated can be utilized for surface strengthening

purpose. The grind hardened component shows that there is a phase transformation of

fenite to pearlite to martensite (figure5.31). SEM reveals that there is no sign of melting

of layer of metals even in the thermally damaged components (figure -5.32).

Figure531 Microscopic views of phase transformation of ferrite to pearlite to martensite

S.No

1

2

3

3

Designation Material

Low carbon casehardening steel ---

Medium carbon hot work steel

Medium carbon spring steel

High carbon non shrinking die steel

% of ' Type of tool Carbon ! used Size

AISI 8620 0.18

AISI H2 I

).IS1 61 50

AISI O2

0.30

0.50

0.90

A1203

A1203

A1203

Diameter =35mm

and Length = 150mm

a) No noticeable microstructural surface alterations (Fine grinding) in AISl 8620

b) Presence of discrete pieces of metals and smeared surface layers i n AISl H21 (no melting of metals)

c) Phase transformed structures a t a grind hardened AISI 1040 steel surface (Grind hardening)

d) Rehardened primary martensitic layer with over tempered Sub- surface zone in AISI TI (Thermal damage with visible burn)

Figure 5.32 SEM photographs of grounded components

5.7 Results and discussion

The comparative study shows that machine hardening is a suitable alternative to

conventional surface modification processes. Further. Grind hardening is more effective

than turn hardening in all aspects. To justify this statement a detailed n~icrostiuctui-a1

analysis, microhardness analysis is presented here.

5.7.1 hIicrostructura1 analysis

Microstructural analysis of a finished surface can provide important information

regarding material properties, reliability and nature of machining. The photomicrographs

of the grind hardened specimens are studied The hardness at depth beneath the surface

and total hardness penetration depth for the various materials also determined and

reported. From the microstructures of Low carbon steel AISI 33 10 and nledium carbon

steels AISI H21. AISI 1040, AISI 4140 and .4ISI 9255 (Figures 5.2. 5.3, 5.4, 5 . 5 and 5.6)

it is evident that the bulk of chip produced during machilung has etched darkly but white

etched bands are also present. This means that the carbide atoms are almobt fully

segregated all along the ferritic matrix, i.e.. ma~-tensitic structure is fortned which are

visible or~ly at sufficient magnification. Beneath this white layel- is a thin transition zone

in u.hich the prior austenitz grain bou~ldaries etched ~vhite and the grain centers tiark. due

to the presence of Carbon (0.2 to O.5wt. %) ill the materials. The structure consists of

mai-tensite that appears in the foim of parallel needles within the folmer austenitc grains.

The retention of austenite is due to the prese~ice alloying elements like Cr, W, V and Co

etc.

From the microstiuctures o f high carbon steels AISI T4, AISl 1095 .AISI 01.

and AISI D2 (Figures 5.7,5.8.5.9 and 5.10) it is inferred that the bulk csf the cliip

produced during machining has etched darkly bur white etched areas are also present b u ~

dispersed in fewite matrix. Beneath the transition zone there is a dark zone gradually

merging with the stmcture of the substrate. Fulther~ilore, the iirst lath mastensites occul-

forming so called mixed mar-tensite(which are visible at higher illagnification only)

besides the acicular martensite. Generally, this lath i~lartensite will only be obtained if

the carbon content exceeds 0.6wt%.

This etching response suggests that about 800°C has been reached in the bulk of

the chip and about 900°C or more has been reached it1 the white etched bands. which may

be expected as reported by Doyle and Dean (1980). The tenlperatui-e developed at the

surface of the workpiece has an i~iipoi-tant influence in the metal removal. However,

heating and cooling of the surface would occur rapidly and this may have the influence of

producing phase changes in the surface.

5.7.2 Microhardness analysis

The rnicrohardness of the various specimens is found by using Vickers's

microhardness tester. Here. the grinding wheel is assumed to be a moving heat source.

the temperature at the contact zone between the grinding wheel and the work piece is of

very high order, The higher hardness resulted from the outer surface is due to the

hi-~nation of martensite, which is obtained by short t i~ne austenizatio~l of sur.f>ce layers

lvith self-quenching.

The hardness of the ground materials at various depths for different depth of cut

(~vith different number of passes )are shown in figures 5.18. 5.19. 5.20 , 5.2 1.5.22 and

5.23 for the ~naterials XIS1 3310. .4ISI H 21. AISI 1040, AISI I095.AISI OI and AISI

D2 respectively.

The influence of numbel. of passes for obtaining higl~er hardness is shown in

tables 5.18. 5.19. 5.20. 5.21. 5.22 and 5.23 for the material AISI33 10. .41SI H21.

AISI1040. AlSI 1095. AISI O l a ~ i d AISl D2 respectively. Figure 5.23 si~o\\.s the

intluence of number of passes 011 11al.ciness at various numbers of passes.

The hardness obtained on the g ~ ~ u n d specimen compared with turned specimen is

shown in figures 5.1 1. 5.12, 5.13. 5.14. 5.15 and 5.16 hr. the materials AISI 4140, AISI

9255. and AISI T4 respectively.

The hardness obtained on the gas carbul-ized and induction hardened materials are

compared with the nlachine hardened specimens in figure 5.17.

It is inferred from the graphs (Figures 5.14. 5.15, 5.16, 5.18, 5.19, 5.20, 5.2 1 , 5.22

and 5.23) that the total depth of cut!number of passes increases the hardness at the surface

to certain depth of cut only. Further increase in total depth of cut (increase in number of

passes) decreases the hardness. This is due to the increase in total depth of cut after

cel-tain period decreases the specific cutting energy, which inturn decreases the

temperature developed at the surface. Thus. the teinperature at the surface of the work

piece has an important influence on the metal renloval at higher depth of cut. The

increase in total depth of cut above the critical value decreases specific energy (Us).

However, beyond a particular depth of cut the rate of change of pailition energy(R) is

much lesser than the rate of change of specific energy. Hence, the surface temperature

decreases with increase in depth of cut. Thus, for cuts where depth of cut is above the

critical value. increasing the deptii of cut may rfduce the surfact te~nperatul-c. Tlius, in

all cases, the specific energy per unit volu~nc is thc ~najor quantity which drtzl-mines the

level of surface temperature in~o1vt.d in a grinding proccss.

The hardness of the ground sl-recirnen AISI 33 10 at various depths for different

numbers of passes is shown in Figure 5.18. The maximum llardness is between 307VHK

and 309 VHN is obtained at a total depth of cut of 1200pm at the hardness peneti-ation

depth of 1.2mn1.

The l~ardness of tlie ground specimen AISI H21 at various depths foi- different

numbers of passes is shown in Figure 5.19. The inaxirnum hardness is between 371VHN

and 373VHN is obtained at a total depth of cut of 1200pm at the hardness penetration

depth of 1.4mm. AISI H21 is a hot work steel having more chromium, tungsten and

vanadiuln content. This will be tlie reason for higher hardness formation.

The hardness of the ground specimen 41S1 I040 at various depths for different

depth of cut shown in figure 5.20 .The higher 1ia1-dness is obtained at 13"' pass for the

total depth of cut of 1.2 lnln at the hardness penetration depth of 1.21nm.

When comparing, the rnaxinlum hardness for AISI 1040 (C 0.35%) and AISI

4140 (C 0.40%) there is not much variation in the hardness obtained, even though the

percentage of carbon in AISI 4140 is higher. But, there is considerable increase in

hardness penetration depth (1.44mm). This is because of increase in percentage of

chromium (0.9%) in AISI 4140. This increases the hardenability of the material and also

retains the hardness to some extent.

The maximum hardness of the ground material (AISI 9255) is 365 VHN, for a

total depth of cut of 1.2mm. The hardness penetration depth is 1.321nn1.

The maximuill hardness of ground mater~al AISI T4 (C 0.80%) is 386 \'HY for a

total depth of cut of 1.20mm. The hardness penetration depth is l.56mm. This AISI T4 is

High speed steel (C1' 0.8%. V 1.6%. Co 9.5%. blo 0.8%) imparts high hardenability.

Thus. for a total depth of cut of 1.20mm. thc maximum hat.dness penetration depth

obtained is 1.56mm because of higher I~ardenability.

The maximum hardness of the ground material AISI 1095 (C O.C)?/o) is 367 VHN

at 13"' pass for a total depth of cut of 1.20n1m. The hardness penet~xion depth is 1.21121n.

Here, there is a decrease in hardness penetration depth (HPD) and hardness value. even

though the carbon percentage is higher when compared with AISI T4 because of he

absence of other alloying elements like Cs. V. W and Mn etc.

The nlaxilnunl hardness of the ground material AISI 01 (C0.95%) is 387 VHN

for a total depth of cut of 1.20mm. Tile hardness peileti-ation depth is 1.Omm. When

comparing tlie HPD in AISI T4 and AISI 0 1 , the HPD in AISI T4 is more because of

presence of W 18%. V1% and Cr 4.3%. These alloying elenlents in AISI T4 increase the

hardenability quality and also retain higher hardness even at red hot conditions. (i.e.. the

retention of austenite at higher temperatures (9 1 ODC)).

Similarly, the rnaximuill hardness of ground material AISI D2(C 1.7%) is

477VHK at 1 3 ~ " pass for a total depth of clepth cut 1.2111n-1. The HPD is 1.2111111 11ence

there is considerable increase in hardness because of higher percentage of Cr 12%: V

0.1%, \V 0.5% Si and hln. These alloying eleillents not oiily increase the hardenability

and also retain the hardness even at elevated temperatures (9 10°C).

5.7.3 Other factors

Role of surface roughness

On engineering applications, the surface roughness parameters like Ra, Rt and Rz

are important. Hence, the surface roughness values are measured by using Surftest 402

instrument. The specimens are cleaned by using benzene to renlove any dirt and other

foreign matters and then placed on the platform of the Surftest 40'2.

As the stylus is moved over the specimen, the roughness \.slues of Ra, Rt, and Rz are

automatically indicated.

The rougl~ness values for the various materials AISI 33 10. AISI H2 1 . AISI 1040,

AIS 4140. AISI 9255, AISI T4. AISI 1095, AISI 0 1 . and AISI D2 are measured and the

results are plotted. From the graph (Figure 5.25) and tables 5.18 to 5.23 it is infei~ed that

the values are consistent and also at acceptable level (within 0 8 microns).

The effect of feed rate on hardness

The effect of feed rate is also studitlcl for different materials. Increasing feed rates

are generally coiinected with process forces. ~vhich arc. also found for grind hiirdening

process. The influence of feed rate 011 hardness and hardness penetl.atio11 depth For the

materials AISI 4140, AISI 9255, and AISI T4 21-2 sho~,~ti i n iigures 5.26. 5.27 and 5.28.

For the material AISI 4140. the maximum hardness obtained for an infeed of

0.6mnlimin is 218 VHN, maximurn hardness obtained for the infeed of 1.25mmimin is

228 VHK and the maximum hardness obtained for the infeed of 2.3mmimin is 231 VHN.

For the material AISI 9255. the maximum hardness obtained for an infeed of

0.6min/min is 223 VHN, maximum hardness obtained for the infeed of 1.25ininimin is

229 VHN and the rnaximuln hardness obtained for the infeed of 2.3mmImin is 234 VHN.

Similarly, for the material AISI T4 at an infeed of 0.6 nlmimin the illaximunl

hardness obtained at a total depth of cut of 1.2 Inn1 is 290 VHN. For the infeed of 1.25

nnmimin, the maximuin hardness is 330 VHN and the maximum hardness for the infeed

of 2.3inminlin is 323 VHN.

From the above discussion, it is inferred that the at lo~vei- infeed of 0.6mmimin.

the traveling energy is high but due to lower cutting power the extent of hardened layer is

reciuced. The hardness penetration ciep~h i.01. AlSl 4110 is 0.98111m. \Vhc.i.eas, tile

harciness penetration depth for thc material .41SI 9255 and AlSl T1 is 1.26mmi.mi1l.

For higher infeed of 2.3nim/lnin tlie hardness psnetration depth of AISI 4 130 and

XIS1 9255 is 1.2611~11 and for AISI T4, i t is I .40mm. This is duo to highel. infeed. the

cutting power increases but tlie contact time decreases. IS the con~uct tinlc decreases then

the traveling energy also decreases. Thus the extent of hardened layer is reduced.

At moderate or medium infeed of 1.25mm/min the traveling energy and contact

tiins is high, so the hardness penetration depth and hardness is unilbrm. The hardness

penetration deptli of AISI 1040 is 1.26 mill and for AISI M2 i t is 1.4niin (Figure 5.29).

The influence of percentage of carbon and white etched area on hardness

As regards to tlic influence of percentage of cal-boil, higher hardness is obtained

for the matel-ial AISI H2 lwl~en compared to other materials 41SI 33 10. .4ISI 1040. and

AISI 4 140.Similarly. the material A1S1 D2 exhibits higher hardness as compared to other

materials AISI 9255, AISI T 4 AISI 1095, and AISI 0 I (Figure 5.30).

Froin the ~nici-ostructural and mlcrohardness analysis it is evident that at the white

etched area the hardness is increased. This is due to the presence of etchable martensite

in the surface layers. bIoren\.er, it can be seen that grind hardening of material is

reachable at relatively soft grinding conditions. This hardness progression is similar to

surface treatments like laser and induction hardening. The short time metallurgical

effects in grind-hardening are also comparable to those of laser or electron beam

hardening.

Further the metallographic examination (Figure 5.3 1 ) reveals that in this work the

grinding parameters are being optimized to iiiduce fine ma~tensitic phase transfor~ilatioli

in the surface layers of steels as it is achieved by other surface strengthening processes.

SEM stluctures (Figure 5.32) grain flow pattern shows that there is no sign of melting in

grounded components even if it is thri-mally daniaged. Hence, in this study it is pi-zdicted

that there is insufficient timc for n-iclting to occur. I t clearly gives an idea that, in

grinding, high magnitude thermal pulses of \:cry short duration is involved. This may not

meit a layer o f steel of one nlicron thickness from the cl-ystalline to the amoiphous state.

So, melting does not occur on the ground surface.

Concluding I-emarks

*3 The possibility of' int i~~str ia l application oi' the grinding PI-oczss 1.01- surface

strengthening is identified.

4 T11e surface strengthened parts by utilizing the in-pl-ocess energy ycnzl-ated in

grinding are chai-acterized by tine rna~tensitic structure which is obtained by

phase transfolmations (short time austenisation of surtice/sub-surface layers with

self quenching).

4:. Micro and nlacro cracks are not found in the coniponents which are surface

hardened by grinding process.

0:. There is no surface melting o f metal.

CHAPTER Surface Modification and Surface...

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